Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $16.96   9.227%   $18.5249   $18.52   $18.50   $18.50  
  Purchase #2 $52.97   13.745%   $60.2507   $60.25   $60.25   $60.30  
  Purchase #3 $126.86   12.151%   $142.2748   $142.27   $142.25   $142.30  
  Purchase #4 $79.71   11.483%   $88.8631   $88.86   $88.85   $88.90  
  Purchase #5 $244.55   4.266%   $254.9825   $254.98   $255.00   $255.00  
  Purchase #6 $205.41   12.563%   $231.2157   $231.22   $231.20   $231.20  
  Purchase #7 $203.45   6.672%   $217.0242   $217.02   $217.00   $217.00  
  7 purchase totals = $929.91       $1,013.1359   $1,013.12   $1,013.05   $1,013.20